I just read a study which says that ideally, riders should only weigh around 10% of the weight of their horses. That would mean that I’m (with around 53kg) too heavy for my arabian gelding. Which frankly, I find kind of ridiculous. Sure, he’s small and definitely not built for heavy people but that 10% quota? Only children or anorexic people can fulfill this.
I think it’s unreasonable to measure the relationship between rider and horse only by weight. A light rider can hurt a horse far worse than a heavier person only by wrong seat and impact. Also, it depends largely on the musculature of the horse and how it’s being trained, rather than by whom.